1. Field of the Invention
The present invention relates to an image forming apparatus and image processing apparatus.
2. Description of the Related Art
As an exposure apparatus for use in an electrophotographic image forming apparatus such as a printer and a copying machine, an exposure apparatus including a plurality of linearly arranged light-emitting elements and an image forming optical system including a linearly arranged lens array has been put to practical use. The exposure apparatus generally controls the light emission amount of the plurality of light-emitting elements stepwise in accordance with an image signal, and forms an image of exposure light emitted by the light-emitting elements on the surface of the photosensitive member by using the lens array, thereby exposing the surface of a photosensitive member to light.
An electrostatic latent image formed on the photosensitive member by exposure by the exposure apparatus as described above can have unevenness of exposure (unevenness of a density) caused by, for example, variations in light emission amount between pixels or the optical transmission characteristics of lenses. To reduce this unevenness of exposure, Japanese Patent Laid-Open No. 2001-111823, for example, has disclosed a method in which correction data generated in accordance with the unevenness of exposure is prestored in a memory, and the gain of the light emission amount of each light-emitting element is adjusted by applying this correction data to image data for exposure.
As the lens array of the exposure apparatus, a gradient index lens that can singly form an erect image and can be downsized in accordance with the uniformity between lenses is generally used. The gradient index lens has the optical characteristic that an image of incident light is formed by zigzagging the light in the lens. Accordingly, the transmission of light is restricted by an incident pupil and a maximum view angle acting in the incident pupil. More specifically, the gradient index lens has a view range in which the illuminance on an image plane abruptly decreases by an almost inverse-square characteristic with respect to the distance from the lens axis, and becomes zero in the image plane position of the maximum view angle. When compared to an ordinary lens that gradually decreases the light amount by the cos4 characteristic, therefore, the gradient index lens has the feature that the width of a light amount fluctuation caused by the decrease in marginal lumination is large.
When the gradient index lens as described above is used as the lens array of the exposure apparatus, the increase in width of the light amount fluctuation caused by the decrease in marginal lumination can be a main cause of the unevenness of exposure. In addition, if the degree of this unevenness of exposure is large, the tone reproducibility after exposure by the exposure apparatus may deteriorate. The tone reproducibility of the exposure apparatus is defined by a dynamic range obtained by subtracting a range corresponding to the compensation amount of the unevenness of exposure from a dynamic range obtained by the number of quantization steps having a digital value corresponding to the light emission amount of the light-emitting elements. The number of quantization steps having a digital value corresponding to the light emission amount of the light-emitting elements is generally restricted to about eight bits by taking account of, for example, the production cost or the accuracy of hardware used in production. To secure the dynamic range corresponding to tone information of image data by about eight quantization bits, it is necessary to reduce the range corresponding to the compensation amount of the unevenness of exposure by reducing the unevenness of exposure in advance.
As a method of reducing the unevenness of exposure caused by the above-described marginal lumination characteristic of the gradient index lens, a method using double lens arrays as the lens array of the exposure apparatus is known. FIG. 9 shows the difference between illuminance characteristics when using a single lens array 901 and double lens arrays 902 as the lens array of the exposure apparatus. When using double lens arrays as shown in FIG. 9, it is possible to increase the ratio at which the view ranges of the image forming planes of lenses overlap each other. This reduces the decrease in marginal lumination in the image forming plane and the unevenness of illuminance, thereby reducing the unevenness of exposure.
Unfortunately, the above-described prior art has the following problem. To form double lens arrays in the exposure apparatus, not only the use amount of lenses doubles, but also it may become difficult to ensure a given off-axial aberration or the like because the lens center deviates from the optical axis, when compared to a single lens array. Accordingly, the production cost increases when providing, with double lens arrays in order to reduce the unevenness of the exposure intensity, an exposure apparatus that controls the exposure intensity by a predetermined, limited number of quantization bits.